Reliable visibility measurement has recently become of particular concern because of the increasing impact that low visibility occurrences have on a contemporary society. By way of example, visibility is an important consideration to airlines and environmentalists. More particularly, much of the emphasis of visibility measurement in the air transportation industry relates to safety and to the identification and measurement of adverse visibility situations within relatively short distances (i.e. in a visual range of approximately 10 km) from an airport. By way of another example, environmentalists are concerned, because increasing levels of man-made air pollution are having an adverse impact on urban areas as well as vegetation and wildlife in scenic areas throughout the United States.
In an effort to abate the adverse effects created by increasing pollution levels, the United States Congress legislated the Clean Air Act and subsequent Amendments. One purpose of the aforementioned legislation was to both remedy existing degradation and prevent future impairments of visibility in certain prescribed federal areas which were unusually susceptible to the adverse effects of man-made air pollution. Accordingly, certain restrictions were placed on major stationary sources of air pollution, so as to prevent the cumulative emission of pollutants from adversely impacting the visibility of the prescribed areas. By way of example, these aforementioned areas include national parks and wilderness sites in the Western United States.
The Clean Air Act also contemplated the protection of visibility in "integral vistas to the areas specifically prescribed in the Act". That is, visibility protection has also been extended to areas surrounding the national parks and wilderness areas, so as to ensure visitor enjoyment thereof. In order to remedy existing visibility impairment, Congress also introduced the concept of Best Available Retrofit Technology (BART). Hence, a major source of pollution that was found to have an unacceptable impact on visibility was required to clean up its emissions by using the BART techniques. However, as a consequence of the relatively stringent regulations and controls that have beem embodied in the Clean Air Act, a large financial burden has been placed on the major sources of emission. As a result, there has been considerable impetus for the development of visibility measurement instrumentation and visibility modeling techniques, whereby pollution sources could be reliably monitored and the pollution abatement regulations consistently enforced.
One potential problem that may result during compliance with the Clean Air Act relates to the changes in emissions that occur when various emission controls are installed at the source. More particularly, most of the particulate matter and sulfur dioxide are suitably removed by currently available emission control equipment. However, much of the nitrogen oxides produced by high temperature combustion continue to be emitted into the atmosphere, whereat said oxides are converted in part to nitrogen dioxide. Such nitrogen dioxide is gaseous matter that absorbs light in the visible part of the electromagnetic spectrum. Such absorption can undesirably lead to visibility degradation. Therefore, a problem that has heretofore been unresolved in the application of conventional pollution detection methods is to be able to distinguish between particulate scattering and nitorgen dioxide absorption, both of which can cause a reddish brown haze under certain observation conditions. Unfortunately, there are no readily available remote monitoring apparatus in which particulate scattering and nitrogen dioxide absorption can be easily and reliably distinguished.
In general terms, visibility is typically related to the measurement of visual range (i.e. the greatest distance that a person can see), and several devices are known for indicating range. However, degradation of visual range correspondingly causes changes in the coloration of distant targets. Generally, there are three conventional instruments and respective methods currently being utilized in order to obtain an indication of visibility.
One such conventional instrument for obtaining a measurement of visibility is that known as the transmissometer. In general terms, transmissometry is the direct measurement of the attenuation of a light beam by the atmosphere. The transmissometer typically includes a source for generating a beam of light (which may or may not be monochromatic). By way of example, the light beam may be generated from a laser source. A detector is provided to measure the attenuation of the light beam. If the initial intensity of the light is known, the attenuation thereof can be related to the visual range.
The main disadvantage to transmissometry is that the reliability thereof is generally limited to short sight paths. That is, the maximum sensitivity of the transmissometer is typically limited to the observation of atmospheric attenuation over the range of a few kilometers. Another fundamental problem typically encountered in transmissometry is that the measured transmission characteristics of the atmosphere over a relatively short distance do not necessarily represent the transmission characteristics of the atmosphere over a much longer visual range. This result is due in part to atmospheric inhomogeneities along the full visual range of the sight path and because other optical effects, such as multiple scattering, usually manifest themselves over only very long optical paths. Hence, the conventional transmissometer is relatively insensitive to multiple scattering effects, so that a measurement of long visual range is difficult and unreliable. Moreover, as will be known to those skilled in the art, the conventional transmissometer is characteristically susceptible to optical alignment purturbations.
A second conventional instrument for measuring visibility is that known as the nephelometer. In general terms, nephelometry is the measurement of light scattering properties of a small volume of air, which volume is assumed to be a representative sample of the atmosphere, as a whole. Hence, a nephelometer is essentially a point source measuring device. However, and as will be recognized by those skilled in the art, utilizing a nephelometer to obtain an indication of air quality may provide inherently inaccurate measurement results, inasmuch as the localized measurement of a small volume of air is, in reality, not necessarily representative of the atmosphere, as a whole. One well-known type of nephelometer is that known as an integrating nephelometer. The integrating nephelometer typically includes a light source and a detector that are arranged relative to one another, so that the light source illuminates a small volume of air. The light scattered by the air is measured by the detector. Optical filters may be installed to simulate the response of the human eye. However, the nephelometer may not be sufficiently sensitive to multiple light scattering in all observation angles. Since long visual ranges are typically associated with low scattered light levels, the measurement of visual range that is obtained with the conventional nephelometer may be replete with uncertainity.
A third conventional instrument for indicating visibility is that known as a telephotometer. In general terms, telephotometry is the measurement of the difference in contrast between the brightness of a distant target and the brightness of the horizon sky just above the target. In obtaining telephotometric measurements of range, the target is usually assumed to be black. The target brightness is typically caused by atmospheric light scattering between the target and the observer. If the distance to the target is known, the visual range can be calculated from the measured contrast ratio. A telephometer generally includes a telescope fitted with a detector, so as to be capable of measuring the intensity of incident light signals. The telephotometer also includes an arrangement of optical filters in order to determine the wavelength of the light received by the detector. Conventional telephotometers may also use a beam shifter so as to efficiently refocus an incident light beam.
In operation, the respective intensities of the light received from the target and the horizon sky above the target are measured and a contrast ratio is then calculated by making a black target approximation. In practice, however, most targets are not black, and, as a consequence of the black target approximation technique utilized by the conventional telephotometer, the measurements obtained thereby may contain inherent errors regarding target brightness.
In view of the foregoing, those skilled in the art will appreciate that the currently known apparatus and methods for making visibility measurements are limited to observations close to the ground. As has already been pointed out above, nephelometry is a point source measurement, and transmissometry is typically limited to relatively short sight path lengths, so that atmospheric inhomogeneities and multiple scattering effects are not measured. What is more, telephotometry requires the observation of a distant target. Moreover, during typical measurement techniques, telephotometry relies upon the conversion of a contrast ratio into a visual range based upon a black target approximation. Such a black target approximation can introduce errors of unknown magnitude. What is more, none of the conventional measurement instruments described above is capable of distinguishing between light scattering and light absorption. More particularly, a transmissometer and a telephotometer measure both scattering and absorption simultaneously. In its conventional application, the nephelometer measures only light scattering. What is even more, conventional visibility measuring apparatus are adapted to provide a limited indication of visual range. However, none of the conventional visibility measuring apparatus is also capable of concurrently providing an accurate indication of visual quality.
By way of one example, a document which relates the effects of light scattering in the atmosphere to visibility is that entitled "Scattered Radiation in the Atmosphere and the Natural Aerosol", by K. F. Bullrich, Advances in Geophysics, Volume 10, page 99 (1964). However, neither this document nor any other known document discloses a method or apparatus for measuring the polarization characteristics of the daylight sky to provide an accurate indication of the visual quality and visual range.